Musical definition of an accidental?

Is an acidental any black key/sharp/flat, or is it specifically a sharp/flat that isn't noted in the key signature?

Musical definition of an accidental?listen to opera

An accidental is any non-key note appearing in the composition. In the key of E flat, A natural would be an accidental. Sometimes they are of a melodic or ornamental nature, such as a passing tone, and sometimes (let's use that A for an example) they are of harmonic structure - that A natural would appear in the V/V (double dominant chord) in the key of E flat - an F or F7 chord leading a Bb chord. So - it doesnt' have to always be a BLACK key - just non-tonal (not in the key signature).

Musical definition of an accidental?concert venue opera theater

isn't that the symbol that tells you to not play the note as a sharp or flat (so you make it a natural) even though the rest of the same note in the song is sharp/flat.
These are the notes that do not belong to harmony %26amp; regular scale as signified by key signature.

We may even use 'Natural' Signs as accidentals besides 'Sharp' %26amp; 'Flats'. Accidental signs cancels the key signature affect within a bar line.

For example in 'G' Scale, F# is the regular key (1 # scale) but if we use 'Natural' Sign on F line in staff or play 'F' then this natural note is an accidental.

Regards :)

-Guitarmonk-
It is any sharp or flat not noted in the keysignature. But a sharp or flat simply means to raise / lower a note in the key signature by one half step.
I believe it's the latter, and also any natural note that should be sharp or flat according to the key signature. That is, it's a note that isn't in the scale for which the piece is written.

We probably shouldn't be focusing on the written key signature. It would be perfectly possible to use one key signature, then write sharps or flats to force the actual piece into another key. An "accidental" would still be the notes that weren't in the scale of the music.
It is a note that isn't in the key signature. I went to google it for a perfect definition, and apparently it is also used to name the black keys on a keyboard.
The latter. It's a note that isn't in the key signature, so if the key signature calls for c# then natural c would be an accidental also.
Please read this if you can (little small) top right paragraph

Encyclopedia Americana

http://books.google.com/books?id=QV0MAAA...